For reducing the fuel consumption in automotive vehicles it has been attempted to at least partially recuperate the kinetic energy released during braking or coasting (that is, during vehicle travel without supplying fuel to the engine in gear). For this purpose, for example, a heavy flywheel has been provided which, during coasting or braking, was coupled to the vehicle wheels by means of a switchable clutch. As a result, kinetic energy of the moving vehicle could be stored upon acceleration of the flywheel and could be utilized for a subsequent engine start and/or engine acceleration. The disadvantage of such a system resides in the additional significant weight of the flywheel which must be driven by the internal-combustion engine as an additional mass.
Further, in conjunction with the above-outlined flywheel system, it has been attempted to provide an electric motor/generator which could be coupled to the running vehicle wheels by a switchable clutch during coasting and/or braking. As a result, the kinetic energy of the vehicle released during coasting or braking could be converted into electrical energy by switching the motor/generator to operate in the generator mode and storing the generated electric energy in a storage battery. For starting and/or acceleration the motor/generator is switched to operate in the motor mode and is supplied by current from the battery so that the electric energy is again converted into kinetic energy. To be able to store appreciable electric energy, however, large-dimension and thus heavy-weight batteries are required. Therefore, in such a system too, in the normal operation a large additional load has to be moved by the internal-combustion engine.
In both above-outlined systems the internal-combustion engine is so designed that it is capable of supplying the energy requirement for the entire load range of the vehicle, while the recuperating systems merely serve for performing auxiliary functions. Since the internal-combustion engine must be designed for the maximum load, but it operates under maximum load conditions only in exceptional cases, that is, the engine during most of its service life is not operated in the optimum rpm and load range, the achievable fuel economy is far from optimal.
It an object of the invention to provide an improved drive system for a vehicle with which a significant reduction in fuel consumption is feasible while utilizing possibilities to recuperate kinetic energy.
This object and others to become apparent as the specification progresses, are accomplished by the invention, according to which, briefly stated, the drive system for a vehicle includes wheel-carrying first and second axles; a storage battery; and a motor/generator selectively operable in motor or generator modes. The motor/generator is connected to the storage battery and cooperates with one of the axles for applying electric energy to the battery when operating in the generator mode and for applying a driving torque to the axle when operating in the motor mode. The drive system further has first and second internal-combustion engines; a transmission connected to the first engine; a first clutch for selectively connecting the transmission to or disconnecting the transmission from, one of the axles; and a second clutch for selectively connecting the second engine to or disconnecting the second engine from, the transmission.
The invention as outlined above provides an at least three-stage drive system wherein the vehicle may be driven from the motor/generator, when operating in the motor mode, by current taken from the battery. During coasting and/or braking electric energy may be fed back into the battery by switching the motor/generator to operate in the generator mode. If in the motor mode of the motor/generator the given nominal output is exceeded because of the extent of driving energy required by the load, the first (primary) clutch is engaged and thus the first engine is operatively coupled to the same drive axle or to an additional drive axle. Thus, the increased torque requirement then may be met either jointly with the motor/generator in the motor mode or alone by the first engine. By providing a suitable electric regulator between the battery and the motor/generator, it is ensured that in the motor mode electric energy is drawn from the battery only up to a minimum value. If, because of unfavorable load conditions, the electric energy drawn from the battery falls below such a minimum value, the torque requirement for the further load has to be made available by the first internal-combustion engine alone. If a torque is required which is in excess of the given nominal output of the first internal-combustion engine, then the second internal-combustion engine, connected to the first internal-combustion engine by a suitable transmission gearing, is coupled into the power train by the second (additional) clutch. In this manner additional driving energy is available from the second internal-combustion engine. By a proper design and assigned load ranges such an arrangement allows the operation of the two internal-combustion engines in the given load ranges at a smallest fuel consumption range.
Tests have shown that in city driving automotive vehicles are driven at least 50% of the operating time in a xe2x80x9ccruisingxe2x80x9d mode, in which only a small amount of energy is needed by the drive system. An increase of energy input is required only for accelerations and uphill travel. In city driving, because of frequent braking, a significant recuperation of the electrical energy from the kinetic energy is possible since the motor/generator then operates in the generator mode. As a result, in city driving it is feasible to sustain a xe2x80x9ccruisingxe2x80x9d mode for a 1.5-ton vehicle with a 10 kW motor/generator.
For accelerations and uphill travel, the first internal-combustion engine is used which has a higher power than that of the motor/generator. Such a higher power may be, for example, twice the power of the motor/generator. Further, for loads that have additional torque requirements, the second internal-combustion engine is activated which has a power higher than that of the first internal-combustion engine. Thus, according to the invention the motor/generator has a small nominal power and the second engine has a power which is at least as high as the nominal power of the first engine. In case of large loads both engines may be operated simultaneously. It is expedient, however, if the second engine has a nominal power higher than that of the first engine to ensure a desired operational mode in each instance in the range of the smallest fuel consumption. Accordingly, the first and second engines may be coupled into or out of the drive train by suitable clutches.
According to an advantageous feature of the invention the motor/generator is associated with a first vehicle axle while the engines are associated with a second vehicle axle. This arrangement ensures that the motor/generator is continuously in engagement with the drive train, whereas the two engines may be operatively connected to the drive train in accordance with torque requirements.
In accordance with another advantageous feature of the invention the motor/generator and the engines are associated with a single, common vehicle axle. In such an arrangement it is expedient to connect the motor/generator with the transmission gearing. It is an advantage of such a system that the motor/generator too, may be coupled with the vehicle axle via the switchable first (main) clutch. As a modification, it is feasible to provide that one motor/generator is associated with one drive axle of the vehicle and a further motor/generator and the engines are associated with another vehicle axle. In such an arrangement, as noted before, switchable respective clutches are arranged between the drives and the transmission gearing, while the main clutch is positioned between the transmission gearing and the drive axle.